This invention relates to high dielectric constant type ceramic compositions, and more particularly to Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -based high dielectric constant type ceramic compositions which exhibit small temperature dependence of its temperature coefficient of dielectric constant (T.C.C.).
Electrical characteristics which must be considered for dielectric materials include dielectric constant, temperature coefficient of dielectric constant, dielectric loss, bias electric field dependence of dielectric constant, capacitance-resistance product and the like.
In particular, it is necessary that the capacitance-resistance product (CR product) be amply high. A specification of Electric Industrial Association of Japan (EIAJ), RC-3698B, for multilayer ceramic capacitors (chip-type) for electronic equipment stipulates that the CR product be at least 500 M.OMEGA...mu.F at room temperature. It is required to maintain the high CR product even at higher temperatures so that capacitors can be used under even more severe conditions. (For example, the United States Department of Defense, Military Industrial Regulation MIL-C-55681B stipulates a CR product at 85.degree. C. or 125.degree. C.)
Further, it is required that the temperature coefficient of dielectric constant be small. In general, materials having large dielectric constants (K) tend to exhibit large T.C.C. values, and it is required that the ratio of K to T.C.C. be large, i.e., the relative value of the variation in the dielectric constant be small.
In the case of elements of multilayer structure, it is necessary to use internal electrode materials which can withstand even at the sintering temperature of dielectric materials because the electrode layer and the dielectric layer are co-fired. Accordingly, if the sintering temperature of the dielectric precious metals is high, expensive materials such as platinum (Pt) or palladium (Pd) must be used not to react with each other. Therefore, a requirement is that sintering be possible at lower temperatures of the order of 1100.degree. C. or below so that inexpensive metal such as silver (Ag) based alloy can be used.
A known high dielectric constant type ceramic composition is a solid solution containing barium titanate (BaTiO.sub.3) as the base and stannates, zirconates, titanates, etc. It is certainly possible to obtain a composition having a high dielectric constant, but such a composition has problems. If the dielectric constant becomes high, then T.C.C. becomes large. Further, the bias electric field dependence becomes large. Furthermore, the sintering temperature of the BaTiO.sub.3 -type materials is high, being of the order of 1,300.degree. to 1,400.degree. C. Out of unavoidable necessity, expensive precious metals such as platinum and palladium which can withstand high temperatures must be used as the internal electrode materials. Thus, capacitor cost increases with increasing capacitance.
In order to overcome the problems of the BaTiO.sub.3 -based materials, extensive studies are being carried out on a variety of low-firing type compositions. For example, Japanese Patent Laid-Open Pub. No. 57204/1980 discloses a Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -based composition; Japanese Patent Laid-Open Pub. No. 51758/1980 discloses a Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -based composition; and Japanese Patent Laid-Open Pub. No. 21662/1977 discloses a Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 -based composition.
The Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -based composition exhibits the following problems. The change of the CR product due to the sintering temperature is quite large. Particularly, the decreasing of the CR product at a higher temperature such as at 85.degree. C. is large. The Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -based composition requires a relatively high sintering temperature. Further, the Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 -based composition exhibits the following problems. If the CR product is large, then the dielectric constant is small. If the dielectric constant is large, then the CR product is small. Furthermore, the T.C.C. of these materials is superior to that of the barium titanate, but it is insufficient.
Further, Japanese Patent Laid-Open Pub. No. 121959/1980 discloses a composition comprising a solid solution of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and lead titanate wherein if necessary a portion of Pb, less than 10 mol %, is substituted by barium, strontium or calcium. However, the T.C.C. of this composition cannot be said to be sufficient, the T.C.C. of the best composition being -59.8% at a temperature of from -25.degree. to 85.degree. C. Further, Japanese Patent Laid-Open Pub. No. 121959/1980 mentioned above does not describe the CR product which is the most important property of a capacitor material. Then the usefulness as a capacitor material is uncertain.
Further, Japanese Patent Laid-Open Pub. No. 25607/1982 discloses a solid solution of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. However, this publication neither describes the CR product nor T.C.C. Thus, the usefulness of the material as a capacitor material is also uncertain.
Furthermore, Japanese Patent Laid-Open Pub. No. 214201/1983 discloses a composition comprising a solid solution of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 wherein if necessary a portion of lead less than 10 mol % is substituted by barium, strontium or calcium. However, the temperature coefficient of dielectric constant of this material is insufficient, and the temperature coefficient of dielectric constant of the best material is -33% at a temperature of from -25.degree. to 85.degree. C. Furthermore, this publication does not describe the CR product. Thus, the usefulness of the material as a capacitor material is uncertain.
An object of the present invention is therefore to provide a high dielectric constant type ceramic composition having a large dielectric constant with a small temperature coefficient and high CR product thereof.